Handover is a vital part of any mobile communication system. A handover is the process of transferring an ongoing connection of a UE from one cell, referred to as the serving cell, to another cell referred to as the target cell in order to accomplish a transparent service over a larger area. The handover should happen without any loss of data and with as small interrupt as possible.
In LTE only the source network node can make a decision about which cell is the target cell based on UE measurement results, and then the UE will be informed the about the target cell to connect to after it receive the HO command from source network node/cell.
In New Radio, NR, i.e., in radio using beam transmissions the HO procedure is different from LTE in some aspects. HO may now be performed also between beams, and not only between cells. The reference signal for UE to measure on may be a dynamically allocated reference signal, and not the static cell-specific reference signal, CRS. The reference signal is not always on, instead it may be controlled by lean design principles and may only be transmitted when needed. However, similar to LTE handover procedure, a source network node can still only determine which beam/node is the target network node after receiving UE measurement report, and the UE can only know which beam/node to connect to after receiving HO command from source network node.
The problem with the solutions in LTE is that if the UE does not receive the HO command, the UE cannot know which target cell, or target network node, to connect to and the HO procedure will therefore fail.
It should in particular be noted that in New Radio, NR, due to the usage of high gain beamforming, it is possible that the serving radio link quality may deteriorate very fast, especially in higher bands. Therefore, the probability that UE does not receive HO command from source, and thus cannot complete the HO, cannot be ignored.
The proposed technology aims to provide mechanisms that counters some of the problems related to handover in a cellular wireless network. It is a particular aim to provide mechanisms that enables a robust handover even in the case of a bad link quality on the radio link between a network node serving the UE and the UE itself.